Immortality

Immortality refers to the avoidance or reversion of death or an entity that appears to exist forever.

Immortality is one of the defining characteristics of the Q. The fact that the Q Continuum forced immortality onto other Q dismayed one of the Q who would attempt suicide. He claimed that immortality was one of the vulnerabilities of the Q. (VOY: "Death Wish")

The Greek gods were immortal. In order to end their existence, they spread themselves against the wind, becoming thinner until reaching a point where they could not reform into their bodies. (TOS: "Who Mourns for Adonais?")

This heavily implies that the Companion was immortal or near-immortal as well.

The Gideon, in practice, were immortal for awhile between the elimination and reintroduction of germs on their planet, as death came to them only when their body couldn't regenerate itself due to disease. (TOS: "The Mark of Gideon")